Looking at a 75 280 and it needs a battery. Wondering if I can throw the battery from my 240D into it to try starting it. Is this possible?

sure, battery is a battery if the voltage is the same, there were no six or 32 volt cars to my knowleDge other than the old VW six volt systems, IF it will fit into the box and the posts will reach the cables. Yes.

Lots of cars up to the '50s and even the '60s had 6V electricals.

Most MB passenger cars from the mid '60s to the turn of the century, gas or Diesel, use a group 49 or H8 battery.

Soxto
83 300SD
98 E320 wagon

As other members have stated, as long as the voltage is the same; you shouldn't have issues.

I put my '82 240D's old Group 49 battery in my '72 W114 SOHC 250C and it fit just fine. I believe the SOHC gas models have the same oversize battery tray as the diesel models.
With your car, there's no problem with the voltage, but the twin-cam 280 engine is bulkier, with the intake on the driver's side. The battery tray on yours may be smaller or located different.

Happy Motoring, Mark

__________________
DrDKW

You can use torch batteries , thats if you have enough of them ..lol